1. Field of the Invention
The invention relates in general to the fabrication of an integrated circuit component, and more particularly to the fabrication of a flash memory designed to miniaturize the dimensions of a memory component.
2. Description of the Related Art
An electrical erasable programmable read only memory (EEPROM) is a memory component that is widely used in personal computers and electronic equipment. A conventional EEPROM memory cell has a floating gate transistor structure and has the ability to be programmed, to be erased, and to store data while connected in a circuit. These abilities give EEPROMs advantages over other types of memory. However, EEPROMs suffer from drawbacks, such as a slow storage and retrieval time (about 150 to 200 ns). Recently, a type of EEPROM with a faster storage and retrieval time (about 70 to 80 ns) has been developed, and has been coined "flash" EEPROM memory by Intel Corporation of the United States.
A conventional EEPROM has a floating gate transistor structure as shown in FIG. 1. The floating gate transistor includes a floating gate layer 10, a controlling gate layer 12, a tunnel oxide layer 14, a dielectric layer 15, a drain region 16, and a source region 18. Its operation is based on the so-called Fowler-Nordheim tunneling effect, in that electrons from the drain region 16 tunnel through the tunnel oxide layer 14 and eventually arrive and are stored in the floating gate layer 10. This has the effect of raising the threshold voltage in the floating gate transistor and enables data storage by the transistor. Alternatively, when the situation is reversed, such that the electrons in the floating gate layer are drawn out through the tunnel oxide layer into the source region 18, data is erased. The aforementioned EEPROM floating gate transistor structure has definite limitations when applied to a densely packed IC circuit. Furthermore, using the same tunnel oxide layer for both data programming and data erasure, as in this design, increases the wear rate of the transistor due to the bi-directional flow of electrons.